Method and control system for applying defined tensile forces

ABSTRACT

The present invention relates to a method and a control system for applying defined clamping forces in a brake that is electrically operable by means of an actuator and includes a first friction surface (friction lining) and a second friction surface (brake disc) in between which a clearance is allowed, wherein the application of the first friction surface against the second friction surface is determined, and wherein a static correlation exists between its actuating travel or, respectively, the actuator position (j actual ) and the clamping force (F actual ) , and wherein values (F meas , j meas ) of the clamping force as well as of the actuator position measured by means of a clamping force sensor and a position sensor are determined.  
     To optimize the time-consuming initialization of the sensors (clamping force sensor, position sensor) used to determine the clamping force and the actuator position and, respectively, to take into account the non-linear rigidity characteristic of the system in the control concept, the, present invention discloses having the correlation described by a mathematical model whose parameters are determined during actuation.

[0001] The present invention relates to a method and a control system for applying defined clamping forces in a brake that is electrically operable by means of an actuator and includes a first friction surface (friction lining) and a second friction surface (brake disc) in between which a clearance is allowed, wherein the application of the first friction surface against the second friction surface is determined, and wherein a static correlation exists between its actuating travel or, respectively, the actuator position and the clamping force, and wherein values of the clamping force as well as of the actuator position are determined, the values being measured by means of a clamping force sensor and a position sensor.

[0002] International patent application WO 96/033010 discloses a disc brake that is preferably operable by means of an electric motor by way of a reducing gear system. The special features of the prior art brake involve that the rotor of the electric motor has an annular design and embraces the reducing gear system in a radial way. These measures allow considerably shortening the axial overall length of the actuating unit. The above-mentioned publication, however, does not provide any indication about how defined clamping forces can be applied in the operation of the state of the art brake.

[0003] The publication ‘Electromechanical Brake System: Actuator Control Development Systems’, SAE Technical Paper Series 970814, discloses a control system for the application of defined clamping forces in an electrically operable brake which is provided by a cascaded arrangement of several controllers. A clamping force sensor is provided to determine the actual value of the clamping force applied by the actuator, while a position sensor is used to determine the actuator position or the actuator speed, respectively. The offset value of the clamping force sensor is determined with the brake opened and then subtracted from the clamping force sensor value during the normal operation. In the prior art control system, the distance between the first friction surface (friction lining) and the second friction surface (brake disc) in the initial position of the system, meaning the so-called air gap, is determined indirectly in such a way that the position is chosen as a contact position and, respectively, as the position sensor offset, in which position the clamping force sensor value that is cleared of the offset falls under a predetermined threshold value.

[0004] The time-consuming initialization of the sensors (clamping force sensor, position sensor) used to determine the clamping force and the actuator position is considered as a shortcoming in the prior art control system. Further, the control concept does not take the non-linear characteristic curve of rigidity of the system into account.

[0005] Therefore, an object of the present invention is to disclose a method and a control system permitting a reduction of the time necessary for the initialization of the sensors. To this end, it is appreciated for the actuator position to be determined in which the brake linings are just abutting on the brake disc, however, with brake force not yet transmitted at this moment. Another objective is to improve the control quality by taking the non-linear characteristic of rigidity into consideration.

[0006] According to the present method, this object is achieved in describing the correlation by a mathematical model whose parameters are determined during actuation.

[0007] To render the idea of the present invention more precise, it is arranged for that the correlation is illustrated by a polynomial of second order F_(estimat)=a₂*φ²+a₁*φ+a₀.

[0008] In a favorable aspect of the method of the present invention, the parameters of the polynomial are determined by means of the method of least squares.

[0009] It is especially advantageous that a recursive algorithm is used for determining the parameters.

[0010] In addition, it is especially suitable to weight the measured values with a forgetting factor for determining the parameters.

[0011] To determine the brake application the first derivative of the polynomial is evaluated in relation to the actuator position, with the actuator position being determined in which the first derivative of the polynomial in relation to the actuator position adopts the value zero.

[0012] With the parameters of the polynomial available, the functioning of the clamping force sensor can be tested in a simple fashion. To this end, e.g. the difference between the current clamping force sensor value and the clamping force value which results from the evaluation of the polynomial for the current actuator position is calculated, and the amount is checked with respect to a threshold value. If this difference exceeds the predefined threshold value, this indicates a defective clamping force sensor.

[0013] If the clamping force sensor is defective (this may possibly occur also with other monitoring provisions such as a mechanism for monitoring the exceeding of a limit value), it is possible to calculate a clamping force substitute value by means of the polynomial identified so far and the measured current actuator position, said substitute value safeguarding at least an emergency operation of the clamping force control.

[0014] The control system of the present invention for implementing the method explained hereinabove is generally comprised of:

[0015] a) a first controller (force/travel controller) to which signals representative of a clamping force nominal value and a clamping force actual value as well as signals representative of an actuator position actual value are sent as input quantities, the output quantity of said controller corresponding to an actuator speed nominal value;

[0016] b) a second controller (speed controller) that is connected downstream of the first controller and to which signals representative of the actuator speed nominal value and an actuator speed actual value are sent as input quantities, the output quantity of said controller corresponding to an actuator torque nominal value; as well as

[0017] c) a third controller (current controller) to which signals representative of the actuator torque nominal value and an actual value of the current to be supplied to the actuator are sent as input quantities, the output quantity of said controller representing a correcting variable for adjusting the actual value of the current to be supplied to the actuator.

[0018] The control system of the present invention is favorably characterized by the provision of a characteristic curve identification module, to which signals representative of the actual value of the position measured at the actuator as well as of the actual value of the clamping force measured at the actuator are sent as input quantities, the output quantities of the module representing signals representative of a clamping force sensor offset value, a position sensor offset value as well as the identified parameter of the polynomial, wherein the signal representative of the clamping force actual value is generated by subtracting the clamping force sensor offset value from the signal representative of the actual value of the clamping force measured at the actuator, and the signal representative of the actuator position actual value is generated by subtracting the position sensor offset value from the signal representative of the actual value of the position measured at the actuator.

[0019] In a favorable improvement of the control system of the present invention, the parameters of the identified polynomial are sent to the first controller for the purpose of linearization of the overall system.

[0020] Further details, features, and advantages of the present invention can be seen in the following description of an embodiment by making reference to the accompanying drawings showing a design of a control loop for implementing the method of the present invention.

[0021] In the drawings,

[0022]FIG. 1 is a design of a control system for implementing the method of the present invention.

[0023]FIG. 2 is the design of a characteristic curve identification module used in the control system of FIG. 1.

[0024]FIG. 3 is the design of a calculation module contained in the characteristic curve identification module of FIG. 2 and used for calculating the designed controlled quantities.

[0025]FIG. 4 is a diagram view of a characteristic curve for explaining the method of the present invention.

[0026] The control system illustrated in FIG. 1 is generally comprised of a first controller or force/travel controller 1, a characteristic curve identification module 8 connected upstream of the force/travel controller 1, a second controller or speed controller 2 connected downstream of the force/travel controller 1, as well as a third controller or current controller 3 connected downstream of the speed controller 2, with an integrated servo booster generating a voltage U that is applied to an actuator 4 (only represented) of an electromechanically operable brake. The actuator 4 is preferably equipped with a clamping force sensor 5, a position measuring system 6 and a current sensor 7, with an output signal F_(meas) of the clamping force sensor 5 representative of the measured clamping force being sent as a first input quantity to the characteristic curve identification module 8 and an output signal φ_(meas) of the position measuring system 6 representative of the measured actuator position being sent as a second input quantity to the characteristic curve identification module 8. The signal I_(actual) of the current sensor 7 that corresponds to the actuator current actual value is sent as an input quantity to the current controller 3 mentioned hereinabove.

[0027] Further, it can be taken from FIG. 1 that a first output quantity F₀ of the characteristic curve identification module 8 that corresponds to a force sensor offset value is subtracted in a first subtraction point 9 from the above-mentioned measured clamping force F_(meas), and the result of the subtraction, meaning a clamping force actual value F_(actual), is sent as a first input quantity to the force/travel controller 1. A second output quantity φ₀ of the characteristic curve identification module 8, which quantity corresponds to a position offset value, is subtracted in a second subtraction point 10 from the output signal φ_(meas) representative of the above-mentioned measured actuator position, and the result of the subtraction, meaning an actuator position actual value φ_(actual), is sent as a second input quantity to the force/travel controller 1. Besides, the gradient of the characteristic curve dF/dφ=F′ is calculated in the characteristic curve identification module 8 and sent as a third input quantity to the force/travel controller 1, while a predetermined clamping force nominal value F_(nominal) predetermined by the vehicle operator represents a fourth input quantity of the force/travel controller 1.

[0028] The input quantities of the speed controller 2 are the output quantity of the force/travel controller 1 that corresponds to the actuator speed nominal value n_(nominal), and an actuator speed actual value n_(actual) which is produced in a differentiator 11 by differentiation of the above-mentioned signal φ_(meas) that corresponds to the measured actuator position as a function of time. The output quantity of the speed controller 2 corresponds to a nominal value M_(nominal) of the brake torque generated by actuator 4.

[0029] The mode of operation of the control circuit illustrated in FIG. 1 and, respectively, the implementation of the method of the present invention will be explained in detail in the following text by making reference to the accompanying drawings of FIGS. 2 to 4.

[0030] As has already been mentioned, FIG. 2 shows the design of the characteristic curve identification module 8 with the input quantities F_(meas), φ_(meas). Both scalar values F_(meas), φ_(meas) are sent to a vector forming element 15 wherein a vector X is formed from the values and sent to a switch 18. Additionally, the signal φ_(meas) that corresponds to the measured actuator position is differentiated in a second differentiator 16 as a function of time, and the differentiated value dφ/dt=φ is sent to an amount computing element 17. The absolute value |φ| is compared in a comparator 19 with a freely selectable threshold value φ_(max) of symbolic notation. When the comparison executed in comparator 19 shows that the inequation |φ|≦φ_(max) is satisfied, the above-mentioned measurement vector X is conducted by the switch 18 and sent to a memory 20 for the intermediate storage of X-values. It is achieved thereby that only measurement vectors are included in the estimation that indicate a relatively slow movement of the brake actuator. This is because the characteristic curve pictures a static correlation that possibly permits an only very inexact estimation with rapid movements of the actuator.

[0031] The above-mentioned quantities F₀, F′, φ₀ as well as a parameter vector P_(estimat,new) is calculated in a calculation module 21 from the measurement vectors X1 stored in memory 20 by means of a recursive estimating process. The parameter vector P_(estimat,new) is separated into parameters a₂, a₁, a₀ in a scalar producing element 22, said parameters being taken into account for producing a polynomial of second order a₂φ²+a₁φ+a₀ illustrating the estimated clamping force F_(estimat). The corresponding terms are produced in multipliers 23, 24, 25 for this purpose and sent to a summing element 26 having an output quantity F_(estimat) that is subtracted from the above-mentioned quantity F_(meas) in a subtraction point 27 for producing a clamping force difference value ΔF. The clamping force difference value ΔF can be used to monitor the clamping force sensor, while F_(estimat) is used as a clamping force substitute value when the clamping force sensor fails.

[0032]FIG. 3 shows the design of the calculating module 8 as illustrated in FIG. 2, with the stored measurement vector X1 as input quantity. The measurement vector X1 is initially sent to a scalar producing element 28 wherein it is separated into two scalar values F_(1meas) and φ_(1meas). While the scalar value F1 _(meas) is sent directly to a subtraction point 29, the signal φ_(1meas) is used to produce a polynomial of second order a₂φ_(1meas) ²+a₁φ_(1meas)+a₀ representative of the estimated clamping force F1 _(estimat), with the parameters of the polynomial being obtained by factorizing a parameter vector P_(estimat,old) which is achieved by shifting the above-mentioned parameter vector P_(estimat,new) ‘backwards’ by a scanning step what is executed in a time delay element 30. As the calculation of the mentioned polynomial corresponds to the procedere described with respect to FIG. 2, there is no need to explain it once more at this point. The scalar value F_(1estimat) is subtracted from the above-mentioned quantity F_(1meas) in the subtraction point 29, with the result of the subtraction, meaning the clamping force difference value ΔF₁, being sent as an input quantity to a correction factor computing element 31 wherein a correction factor of the parameter vector is calculated in such a way that an optimally good consistency of the estimation with the measurement is achieved in the sense of the method of least squares of errors. The output quantity ΔP of the correction factor computing element 31 is added to the parameter vector P_(estimat,old) in an addition point 32 to produce the above-mentioned parameter vector P_(estimat,new) which is processed in a parameter conversion module 33 to achieve the quantities F₀, F′, φ₀ explained in connection with FIG. 2.

[0033]FIG. 4 eventually shows a diagrammatic view of the polynomial defining the estimated clamping force F_(estimat). The individual points X₁₁ to X₁₅ correspond to the vector X1 referred to hereinabove at different values of the actuator position and the clamping force, while the plotted values φ₀ and F₀, respectively, exhibit the position sensor offset value and the clamping force sensor offset value, respectively. 

1. Method of applying defined clamping forces in a brake that is electrically operable by means of an actuator and includes a first friction surface (friction lining) and a second friction surface (brake disc) in between which a clearance is allowed, wherein the application of the first friction surface against the second friction surface is determined, and wherein a static correlation exists between its actuating travel or, respectively, the actuator position (j_(actual)) and the clamping force (F_(actual)), and wherein values (F_(meas), j_(meas)) of the clamping force as well as the actuator position are determined, the values being measured by means of a clamping force sensor and a position sensor, characterized in that the correlation is described by a mathematical model whose parameters are determined during actuation.
 2. Method as claimed in claim 1, characterized in that the correlation is represented by a polynomial of second order F_(estimat)=a₂*φ²+a₁*φ+a₀.
 3. Method as claimed in claim 2, characterized in that the parameters (a₂, a₁, a₀) of the polynomial are determined by means of the method of least squares.
 4. Method as claimed in claim 3, characterized in that a recursive algorithm is used for determining the parameters.
 5. Method as claimed in claim 4, characterized in that the measured values are weighted with a forgetting factor for determining the parameters.
 6. Method as claimed in any one of claims 2 to 6, characterized in that the first derivative of the polynomial in relation to the actuator position is evaluated for detecting the brake application.
 7. Method as claimed in claim 7, characterized in that the actuator position is determined in which the first derivative of the polynomial in relation to the actuator position adopts the value zero.
 8. Method as claimed in any one of claims 2 to 5, characterized in that a clamping force comparison value is constantly produced by means of the identified polynomial, and a clamping force sensor error is inferred from an excessive difference from the measured value.
 9. Method as claimed in any one of claims 2 to 5, characterized in that a clamping force substitute value for the clamping force control is determined by means of the identified polynomial when a clamping force sensor fails.
 10. Control system for applying defined clamping forces in a brake that is electrically operable by means of an actuator and includes a first friction surface (friction lining) and a second friction surface (brake disc) in between which a clearance is allowed, wherein the application of the first friction surface against the second friction surface is determined, and wherein a static correlation exists between its actuating travel or, respectively, the actuator position (j_(actual)) and the clamping force (F_(actual)), including a clamping force sensor for determining the clamping force (F_(actual)) as well as a position sensor for determining the actuator position (j_(actual)), with a first controller (force/travel controller) to which signals representative of a clamping force nominal value (F_(nominal)) and a clamping force actual value (F_(actual)) as well as an actuator position actual value (j_(actual)) are sent as input quantities, the output quantity of said controller corresponding to an actuator speed nominal value (n_(nominal)), a second controller (speed controller) that is connected downstream of the first controller and to which signals representative of the actuator speed nominal value (n_(nominal)) and an actuator speed actual value (n_(actual)) are sent as input quantities, the output quantity of said controller corresponding to an actuator torque nominal value (M_(nominal)), as well as a third controller (current controller) that is connected downstream of the second controller and to which signals representative of the actuator torque nominal value (M_(nominal)) and an actual value (I_(actual)) of the current to be supplied to the actuator are sent as input quantities, the output quantity of said controller representing a correcting variable (U) for adjusting the actual value (I_(actual)) of the current to be supplied to the actuator, characterized in that a characteristic curve identification module (8) is provided, to which signals representative of the actual value of the position (j_(meas)) measured at the actuator (4) as well as of the actual value of the actuating force (F_(meas)) measured at the actuator (4) are sent as input quantities, the output quantities of the module representing signals representative of a force sensor offset value (F₀), a position sensor offset value (j₀) as well as the gradient of a characteristic curve illustrating the correlation, wherein the signal representative of the clamping force actual value (F_(actual)) is generated by subtracting the clamping force sensor offset value (F₀) from the signal representative of the actual value of the clamping force (F_(meas)) measured at the actuator, and the signal representative of the actuator position actual value (j_(actual)) is generated by subtracting the position sensor offset value (j₀) from the signal representative of the actual value of the position (j_(meas)) measured at the actuator.
 11. Control system as claimed in claim, characterized in that the parameters of the identified polynomial are sent to the first controller for the linearization of the characteristic curve of rigidity of the brake. 